1. Field of the Invention
The present invention relates to an extruded styrenic resin foam and a method for producing the same, wherein the base resin composing the styrenic resin extruded foam is a styrenic resin mixture composed of a styrene-(meth)acrylic ester copolymer, or a combination of a styrene-(meth)acrylic ester copolymer and polymethyl methacrylate, and a styrenic resin, and the styrenic resin mixture contains a (meth)acrylic ester component at a ratio of 4 to 45% by weight with reference to the styrenic resin mixture. The present invention provides an extruded styrenic resin foam having an apparent density of 20 to 60 kg/m3, a thickness of 10 to 150 mm, and a low thermal conductivity, and keeping excellent heat insulation performance over a long period of time and flame retardancy, even when the extruded styrenic resin foam is foamed using a blowing agent having an ozone depleting potential of 0 (zero) and a low global warming potential.
2. Relevant Prior Art
Styrenic resin foams have excellent heat insulating properties and favorable mechanical strength. Therefore, styrenic resin foams in the form of board having a certain width have been widely used as insulating materials. Such a foam is usually produced as follows: a styrenic resin material is heated and molten in an extruder, the melt is mixed with a physical blowing agent to make a foamable molten resin composition, the foamable molten resin composition is extruded and foamed through, for example, a slit die provided at the tip of the extruder into a low-pressure region, and, as necessary, the foam is molded by a shaping device connected to the outlet of the die.
Chlorofluorocarbons (hereinafter referred to as CFCs) such as dichlorodifluoromethane has been widely used as blowing agents for producing the styrenic resin extruded foam. However, CFCs are prone to deplete the ozone layer, so that CFCs have recently been replaced with hydrogenated chlorofluorocarbons (hereinafter referred to as HCFCs) having low ozone depleting potentials. However, the ozone depleting potentials of HCFCs are not 0 (zero), so HCFCs are not without risk of depleting the ozone layer. Accordingly, hydrofluorocarbons (hereinafter referred to as HFCs) having an ozone depleting potential of 0 (zero) and no chlorine atom within the molecule thereof have been studied for possible use as blowing agents.
HFCs are preferred from the viewpoint of ozone depleting potentials, but HFCs have high global warming potentials, so that there is still room for improvement from the viewpoint of global environmental protection. Therefore, methods for producing styrenic resin foams using an eco-friendly blowing agent having an ozone depleting potential of 0 (zero) and a low global warming potential have been studied.
For example, isobutane and isopentane used as blowing agents for making dstyrenic resin foams have an ozone depleting potential of 0 (zero) and low global warming potentials, so that they are favorable blowing agents from the viewpoint of global environmental protection. However, isobutane and isopentane in gaseous form have a lower thermal conductivity than air, but have a higher thermal conductivity than chlorofluorocarbons such as CFCs, HCFCs, and HFCs. Therefore, a form containing isobutane or isopentane cannot achieve equivalent heat insulating properties to foam containing an equimolar amount of a chlorofluorocarbon. The heat insulating properties can be improved by increasing the content of isobutane or isopentane in the foam, but isobutane and isopentane are so flammable that it is very difficult to impart sufficient flame retardancy to the resultant foam. The permeation rates of isobutane and isopentane through a styrenic resin are much lower than that of air, but higher than that of CFCs, so that isobutane and isopentane gradually dissipate from the foam. On this account, the thermal conductivity of the foam gradually increases. Accordingly, it has been difficult to produce a foam having long-lasting heat insulating properties and flame retardancy with the use of isobutane or isopentane as a blowing agent.
It has been also studied to use carbon dioxide and water, which have an ozone depleting potential of 0 (zero) and a lower global warming potential than hydrocarbon, as the main ingredients of a blowing agent, but carbon dioxide and water quickly dissipate from the foam, so that the resultant foam cannot achieve high heat insulating properties.
As a method for producing an extruded styrenic resin foam having excellent environmental suitability and keeping high heat insulation performance and high strength, for example, WO 99/33625 discloses a method for producing a foam having excellent heat insulating properties and excellent compression strength by extrusion foaming, wherein a blowing agent is injected into a styrenic resin, and the blowing agent is as environmentally safe as possible, and is composed mainly of less than 40% by weight and 5% by weight or more of ether, and more than 60% by weight and 95% by weight or less of a saturated hydrocarbon having 3 to 5 carbon atoms, which may be partially replaced with a hydrofluorocarbon, with reference to the whole amount of the blowing agent.
WO 99/54390 discloses a method for producing a foam having a cell structure containing large and small cells, and specified the cell form through the use of a blowing agent composed of 40% by weight or more and 85% by weight or less of one ether selected from dimethyl ether, methyl ethyl ether, methyl ethyl ether, and diethyl ether, and 15% by weight or more and 60% by weight or less of one saturated hydrocarbon having 3 to 5 carbon atoms with reference to the whole amount of the blowing agent.
WO 02/051918A1 discloses a foam and a method producing the same, wherein the blowing agent contains at least one saturated hydrocarbon having 3 to 5 carbon atoms, and at least one compound selected from a group consists of halogen flame retardants, tetrazole compounds, specific nitrogen-containing compounds, metal borates, and boron oxide, and the cell structure composing the foam satisfies a cell anisotropy represented by a specific formula.
However, the foams produced by the above-described methods exhibits high heat insulating properties immediately after the production, but their heat insulation performance gradually deteriorates as the hydrocarbon gradually dissipates from the foams. Therefore, they still have problems in maintenance of high heat insulation performance over a long period of time.
Japanese Patent Application Laid-Open (JP-A) Nos. 2006-131719 and 2006-131757 proposed methods including addition of a gas barrier resin to a styrenic resin. JP-A-2006-131719 proposed a method including addition of a nitrile resin as a gas barrier resin. JP-A-2006-131757 proposed a method for extrusion foaming, wherein a vinyl alcohol resin is used as a gas barrier resin, and the resin is extruded and foamed in the presence of a compound having a molecular weight of 1000 or less and one or more hydroxy groups within one molecule thereof.
Gas barrier resins prevent the dissipation of low thermal conductive blowing agents such as isobutane from foams, and retard the inflow of air into foam cells thereby improving heat insulating properties immediately after the extrusion. However, because gas barrier resins inhibit foaming, it is difficult to produce foams with low apparent density. Even if isobutane slowly dissipates from a foam and the inflow rate of air into the cells is low, the content of isobutane decreases and air flows in the cells after a lapse of long period, which hinders the maintenance of heat insulating properties over a long period of time.
Various methods for producing styrenic resin extruded foams are reported, wherein a blowing agent having an ozone depleting potential of 0 (zero) and a low global warming potential is used to produce an extruded styrenic resin foam having excellent flame retardancy and providing a low thermal conductivity and excellent heat insulating properties over a long period of time.
The inventors have extensively studied the method for producing an extruded styrenic resin foam having excellent flame retardancy and keeping a low thermal conductivity and excellent heat insulating properties over a long period of time, even when a blowing agent having an ozone depleting potential of 0 (zero) and a low global warming potential is used, and have found that an extruded styrenic resin foam providing a low thermal conductivity and excellent heat insulating properties over a long period of time is produced with the use of a styrenic resin mixture composed of a styrene-(meth)acrylic ester copolymer, or a combination of a styrene-(meth)acrylic ester copolymer and polymethyl methacrylate, and a styrenic resin (meth) acrylic ester as the base resin composing the extruded styrenic resin foam, wherein the styrenic resin mixture contains a (meth)acrylic ester component at a specific ratio.